1. Field of the Invention
The present invention relates to a gas concentration detector including a gas sensor and an abnormality detecting section for detecting abnormality in the gas sensor.
2. Description of Related Art
There is known a gas concentration detector for detecting concentration of NOx contained in exhaust gas of an automobile, for example, the gas concentration detector including a gas sensor and an abnormality detecting section for detecting abnormality in the gas sensor. For example, refer to Japanese Patent No. 4020019.
The gas sensor of this gas concentration detector includes two plate-like solid electrolyte bodies having oxygen ion conductivity. Each solid electrolyte body is formed with electrode on both surfaces thereof. Cells are formed by the solid electrolyte bodies and the electrodes. The cells include a pump cell for adjusting an oxygen concentration in a measurement gas, a monitor cell for detecting the oxygen concentration in the measurement gas and a sensor cell for detecting concentration of a specific gas contained in the measurement gas. This gas concentration detector is configured to measure a current flowing to the monitor cell and a current flowing to the sensor cell, and calculate the concentration of the specific gas based on the measured values of theses currents.
The electrodes are connected with wires through which the currents flow. Therefore, if any one of the wires breaks for some reason, it becomes impossible to accurately measure the concentration of the specific gas. Accordingly, the gas concentration detector has to periodically check whether there is breakage in any one of the wires.
To check wire breakage, an AC voltage is applied to each of the cells through the wires. As explained in the foregoing, each cell has the structure as a capacitor in which electrodes are formed on both surfaces of a solid electrolyte body. Accordingly, if there is no wire breakage, an AC current flows to the cell when the AC voltage is applied to the cell, while on the other hand, if there is wire breakage, no AC current flows to the cell even when the AC voltage is applied to the cell. Hence, by detecting whether the value of the AC current flowing to the cell when the AC voltage is applied to the cell is larger then a predetermined threshold value or not, it is possible to determine presence or absence of wire breakage.
However, in the above gas concentration detector, the cell formed in one of the two solid electrolyte bodies is electrically insulated from the cell formed in the other solid electrolyte body. Therefore, to determine presence or absence of wire breakage for all the wires, it is necessary to apply the AC voltage to each of the cells individually. Accordingly, since a circuit for applying the AC voltage has to be provided for each of the cells, the circuit scale and cost of the abnormality detecting section increase. In addition, since the AC voltage is applied to the cells successively, it takes time to complete the wire breakage check.